1. Field of the Invention
The invention is in the field of mass spectrometry and more specifically in the field of ionization sources in mass spectrometry.
2. Related Art
Laser-based ionization techniques, which include laser desorption/ionization (LDI) and matrix-assisted laser desorption/ionization (MALDI), are useful tools for mass spectrometric analysis. These techniques involve irradiating a sample containing an analyte substance with a short pulse of radiation, typically emitted by a laser. The radiation is absorbed by the sample, resulting in the desorption and ionization of analyte molecules from the sample. In the MALDI process, the sample is prepared by diluting small amounts of the analyte substance in a large molar excess of matrix material, which is highly absorbent at the irradiation wavelength and which assists in the desorption and ionization of the analyte molecules. MALDI is a particularly useful technique for the analysis of large biological molecules, such as peptides or proteins that may undergo fragmentation when subjected to alternative ionization methods. Furthermore, MALDI tends to produce singly-charged ions, thereby facilitating interpretation of the resultant mass spectra. The ions produced by the LDI or MALDI source (or product ions derived therefrom) may be analyzed using any one or combination of mass analyzers known in the art, including quadrupole mass filters, quadrupole ion traps, time-of-flight analyzers, Fourier transform ion cyclotron resonance cells, and electrostatic traps.
Recently, there has been growing interest in the use of LDI/MALDI mass spectrometry to generate spatially resolved maps of analyte concentrations in a biological material, such as a tissue sample. This process, which is often referred to as mass spectral tissue imaging, offers great promise as a tool for the study of drug absorption and excretion by selected tissues. Because analyte concentrations in a tissue sample may exhibit large spatial gradients, it is generally desirable to perform tissue imaging experiments at high spatial resolution in order to gain useful information regarding analyte concentration profiles at areas of interest within the sample.
The minimum spatial resolution that can be obtained using a MALDI or LDI source will be partially determined by the spot size, i.e., the area of the sample that is irradiated by the laser or other irradiation source. In most commercially available MALDI sources, the spot size has a diameter of around 100 μm, which is too large for many tissue imaging applications. The spot size may be reduced by more tightly focusing the radiation beam at the sample surface, e.g., by using a beam-focusing lens having a shorter focal length with a large aperture. However, the presence and positioning in the ionization source chamber of the ion guide or other optics, which transport the ions from the sample location to the mass analyzer, will often interfere with the placement of a short focal length lens, thereby making it difficult or impossible to focus the beam to the desired size. The placement of a short focal length lens may also be rendered more difficult by the presence of viewing optics employed to acquire an image of the sample.
In addition to the above it can be desirable to position a collection device as close as possible to the point of sample desorption, or at least as close as possible to the direction of flight of the ions. The desire to bring both the collection device and the lens as close as possible to the MALDI sample creates a conflict because there is limited room near the MALDI sample.
One approach to reducing this conflict is to use a lens or other optic having an opening configured for ions to pass through. Such systems work well when the ions are well collimated into a beam, and an ion collection device or the mass analyzer itself can be positioned in line with the path of the laser beam. This approach is most satisfactory where ions are extracted from the source region with a high electric field, thus preventing ions from dispersing before they reach the optic opening. Ions in these systems, typically go from a region of high potential/electric field to the substantially vacuum region of the mass analyzer itself.
In systems without strong extraction fields, e.g., ion traps, quadrupoles, ICR cells, etc., the use of an optic with an opening can be very inefficient because the ions have a greater chance to disperse before reaching the opening. To accommodate such systems, in some MALDI systems ions are generated from a MALDI sample and then collected by a skimmer or other ion collection device for transport to a mass spectrometer. In these systems high pressure in front of skimmer enables the ions that have been dispersed not to dissociate and be efficiently collected by providing the pressure differential between the skimmer and the region prior to the skimmer. In view of the above discussion, there is a need in the art for an LDI or MALDI source that prevents dispersion, and provides for high efficiency of ion collection in tissue imaging or other applications that require the use of systems without strong extraction fields.